Abstract

Microscopic residual bubble nuclei can persist on the order of 1 s following a cavitation event. These bubbles can limit the efficacy of ultrasound therapies such as shock wave lithotripsy and histotripsy, because they attenuate pulses that arrive subsequent to their formation and seed repetitive cavitation activity at a discrete set of sites (cavitation memory). Here, we explore a strategy for the removal of these residual bubbles following a cavitation event, using low-amplitude ultrasound pulses to stimulate bubble coalescence. All experiments were conducted in degassed water and monitored using high-speed photography. In each case, a 2-MHz histotripsy transducer was used to initiate cavitation activity (a cavitational bubble cloud), the collapse of which generated a population of residual bubble nuclei. This residual nuclei population was then sonicated using a 1 ms pulse from a separate 500-kHz transducer, which we term the bubble removal pulse. Bubble removal pulse amplitudes ranging from 0 to 1.7 MPa were tested, and the backlit area of shadow from bubbles remaining in the field following bubble removal was calculated to quantify efficacy. It was found that an ideal amplitude range exists (roughly 180 to 570 kPa) in which bubble removal pulses stimulate the aggregation and subsequent coalescence of residual bubble nuclei, effectively removing them from the field. Further optimization of bubble removal pulse sequences stands to provide an adjunct to cavitation-based ultrasound therapies such as shock wave lithotripsy and histotripsy, mitigating the effects of residual bubble nuclei that currently limit their efficacy.

Commentaires
1

Cavitation bubbles may occur during shock wave lithotripsy and high energy ultrasound application. They may support stone fragmentation in addition to other mechanisms such as spallation (Hopkinsons effect), dynamic fatigue and others and may simultaneously be responsible for tissue lesions by puncture of small vessels. How should we judge cavitation, as beneficial or dangerous? At least, cavitation control would be useful.

Cavitation bubbles are generated by the tensile tail of a shock wave. A following shock wave running through an existing cloud of cavitation bubbles will be significantly attenuated and disturbed. Whereas cavitation bubbles usually collapse before following shock waves are released, small residual bubble nuclei may exist milliseconds or even seconds which, in turn, may affect cavitation of the following shock waves.
The authors remove the persistent nuclei by low amplitude ultrasound and intend to find a technique to increase efficiency of therapies such as lithotripsy and histotripsy.

In vitro results confirm the effect of low amplitude ultrasound on subsequent cavitation events. Next steps could be to further develop this technique for clinical use and to evaluate its clinical impact on lithotripsy eg., which appears to be demanding.

Cavitation bubbles may occur during shock wave lithotripsy and high energy ultrasound application. They may support stone fragmentation in addition to other mechanisms such as spallation (Hopkinsons effect), dynamic fatigue and others and may simultaneously be responsible for tissue lesions by puncture of small vessels. How should we judge cavitation, as beneficial or dangerous? At least, cavitation control would be useful.
Cavitation bubbles are generated by the tensile tail of a shock wave. A following shock wave running through an existing cloud of cavitation bubbles will be significantly attenuated and disturbed. Whereas cavitation bubbles usually collapse before following shock waves are released, small residual bubble nuclei may exist milliseconds or even seconds which, in turn, may affect cavitation of the following shock waves.
The authors remove the persistent nuclei by low amplitude ultrasound and intend to find a technique to increase efficiency of therapies such as lithotripsy and histotripsy.
In vitro results confirm the effect of low amplitude ultrasound on subsequent cavitation events. Next steps could be to further develop this technique for clinical use and to evaluate its clinical impact on lithotripsy eg., which appears to be demanding.